This proposal is designed to test the hypothesis that different types of formulas commercially available for children with PKU have different capacities to support their growth. The investigators will measure growth and nutritional parameters in children aged 2-12 years on one of three different formulas: Phenex-2, Periflex and Phenyl-Free. Children to be studied are already receiving one of these therapies. They will be followed for one year. Enrollment is ongoing.